Deep subduction quakes are big, but this one wasn't the Big One.(The Seattle Times)

SEATTLE _ It was not the Big One scientists expect in this region sometime between now and the end of eternity, but the magnitude of Wednesday's earthquake was relatively unusual.

It was a deep-subduction-zone quake, like those of 1949 and 1965, near where the leading edge of one of the "plates" of the Earth's cool crust dives down into the hot mantle.

In fact, University of Washington geophysicists have pinpointed the location of this quake as nearly identical to that of the 1949 quake, about 30 miles underneath Anderson Island, east of Olympia.

That said, it's difficult to know with certainty if the two quakes were in the same place because the technology in 1949 to determine the size and location of quakes was imprecise compared with today's.

In fact, scientists recently installed a network of 44 "strong motion" stations in the Puget Sound area that are capable of accurately measuring major quakes.

With those instruments in place, "this is a real important earthquake," said Ruth Ludwin, a research scientist for the University of Washington Geophysics Program in Seattle. The strong-motion stations will help geophysicists determine how shock waves traveled through the various geologic formations and what effect they had. Structural engineers can then better anticipate the forces that might rock buildings and bridges.

An explanation of the area's geology is helpful in putting Wednesday's earthquake in context and in understanding why a worse one is inevitable.